Use __attribute__((format(printf, ...))) directly

Instead of defining the macro _R_(), define __attribute__() as a macro for
compilers that do not support __attribute__(), namely Microsoft Visual
Studio.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>

test-tool/iscsi-support.h

@@ -65,7 +65,8 @@ extern int loglevel;
 #define LOG_SILENT  0
 #define LOG_NORMAL  1
 #define LOG_VERBOSE 2
-void logging(int level, const char *format, ...) _R_(2,3);
+void logging(int level, const char *format, ...)
+__attribute__((format(printf, 2, 3)));
 
 /*
  * define special flags for logging a blank line, so compiler